1. Field of the Invention
The present invention relates to an optical recording medium comprising a phase-change recording material which is optically changeable by the application thereto of a light beam so as to carry out recording, reproducing, and overwriting of information, and more particularly to a phase-change optical recording medium capable of recording information with high density at high recording linear velocity.
In addition, the present invention also relates to a recording method and apparatus for the above-mentioned phase-change optical recording medium by a Constant Angular Velocity (CAV) system or a Constant Linear Velocity (CLV) system which is achieved by dividing the recording area of the recording medium into a plurality of sections in the radial direction of the recording medium.
2. Discussion of Background
To reproduce or record information in an optical disc with high density by using a laser beam, there is conventionally known the overwriting mode in which recording marks are formed in the optical disc by the application of a plurality of short-length pulse trains as disclosed in Japanese Laid-Open Patent Application 3-185628. However, the above-mentioned overwriting mode has the drawback that the laser power becomes partially insufficient. More specifically, in the case where the optical disc is rotated at a constant number of revolutions, the relative speed of the laser spot is not constant between a recording area at the inner circumference of the disc and that at the outer circumference thereof, viewed in a radial direction of the disc. In such a case, the laser power becomes insufficient in the area where the relative speed of the laser spot is fast. Furthermore, there will occur a new problem that design of the circuit becomes difficult.
To solve the above-mentioned problems, there is a proposal in Japanese Laid-Open Patent Application 6-12674. This proposal is that when an input signal with a particular wave form, for example, an eight to fourteen modulation signal (EFM signal) is given to an optical disc, the wave form of laser is modulated depending upon the linear velocity. More specifically, the laser is modulated into a short-length train of pulses when the linear velocity is slower than a predetermined value (L0); and the laser is modulated into one pulse which is made slightly shorter than the corresponding recording mark when the linear velocity is faster than the aforementioned value (L0).
There is an increasing demand for development of a phase-change optical recording medium and a recording method therefor, which will be able to achieve high-density recording such that the recording capacity thereof is the same or more than that of DVD-ROM, and attain high-speed recording at a linear velocity of 2 times or more (about 7 m/s or more) that of the nominal speed for the DVD-ROM. However, when such an optical recording medium is subjected to the above-mentioned CAV or CLV recording method, it is conventionally known that good recording characteristics cannot be obtained with respect to jitter value by slightly shortening the input pulse width. On the contrary, when the aforementioned conventional method, as proposed in Japanese Laid-Open Patent Application 6-12674, of modulating laser into a short-length train of pulses at the lower linear velocity side, good results can be produced. It is considered that, in Japanese Laid-Open Patent Application 6-12674, the above-mentioned recording method is employed because the optical disc employs a composition close to a compound of Ge2Sb2Te5.
However, when recording is carried out using the modulated laser with the pulse width being fixed so as not to be deformed at the higher linear velocity side, the pulse width becomes too short in the recording area of the lower linear velocity side, that is, at an inner circumference of the recording medium. In this area, there is a tendency of impairing the jitter value due to insufficient recording power. It is considered that such a phenomenon is also caused by the composition of a recording layer for use in the optical recording medium.